1. Field of the Invention
The present invention relates to a vapor fuel supply control apparatus in a lean-burn internal combustion engine, which supplies an intake system with a vapor fuel produced in, e.g., a fuel tank etc in accordance with an operating state of the lean-burn internal combustion engine.
2. Related Background Art
In an engine which has hitherto generally been used, the fuel is injected out of a fuel injection valve to an intake port, and a combustion chamber is previously supplied with a homogeneous air-fuel mixture of the fuel and the air. In the thus constructed engine, an intake passageway is opened and closed by a throttle valve interlocking with an operation of an accelerator.
A quantity of the intake air supplied to the combustion chamber of the engine (which is resultantly a quantity of a homogeneously mixed gas of the fuel and the air) is controlled by opening and closing the throttle valve, thereby controlling an output of the engine.
According to the technology based on the so-called homogeneous combustion described above, a large intake negative pressure is produced with a throttle operation of the throttle valve, and a pumping loss becomes large, resulting in a decline of efficiency. By contrast, a technology known as a so-called stratified charge combustion, wherein the throttle valve is throttled small, the fuel is supplied directly to the combustion chamber, a combustible air-fuel mixture is thereby made to exist in the vicinity of a spark plug, and an igniting property is enhanced by increasing an air/fuel ratio of the portion concerned. According to this technology, when in a low-load state of the engine, the fuel injected is supplied in dispersion around the spark plug, and the stratified charge combustion is executed with the throttle valve substantially fully opened. The pumping loss is thus reduced, and a fuel consumption is enhanced.
The internal combustion engine capable of performing the above-described stratified charge combustion sequentially assumes, for example, when changed from the low-load to a high-load, combustion states such as the stratified charge combustion, a semi-stratified charge combustion, a homogeneous lean-burn and a homogeneous combustion.
The stratified charge combustion is, as explained before, that the air/fuel mixture exhibiting a low air/fuel ratio is made to exist in the vicinity of the spark plug, and is stratified between this mixture and a gas at another portion.
The semi-stratified charge combustion has a smaller degree of its being stratified than the stratified charge combustion.
The homogeneous lean-burn has a homogeneousity of the fuel and the air but is small in terms of a ratio of the fuel.
The homogeneous combustion has a homogeneous mixture of the fuel and the air and a high ratio of the fuel.
Further, there might be a case where a swirl is formed in the air/fuel mixture of the injected fuel when the above-described stratified charge combustion is conducted, and when the lean-burn is effected. That is, an intake port is provided with a swirl control valve (SCV), and an aperture of this valve SCV is controlled, thereby controlling an intensity of the swirl. As a result, the combustibility is enhanced with a small amount of the fuel supplied.
Incidentally, there is known an apparatus for controlling a supply of a vapor fuel in a lean-burn internal combustion engine (Japanese Patent Application Laid-Open Publication No.4-194354) constructed such that the vapor fuel (vapor) from the fuel tank etc is temporarily accumulated in a canister, and the vapor fuel accumulated is supplied to an intake system in accordance with an operating state of the internal combustion engine.
According to this technology, a purge control valve is provided within a vapor fuel oriented purge passageway through which the canister for adsorbing the vapor fuel is connected to an intake passageway. Then, the purge control valve is controlled so as to obtain a proper fuel purge quantity (a quantity of the vapor fuel introduced into the intake passageway, which is hereinafter simply referred to as a purge quantity) (e.g., so as to supply the vapor fuel if a load of the engine is large) in accordance with the operating state of the engine.
On the other hand, there exists an internal combustion engine in which the air/fuel ratio is detected by use of an air/fuel ratio provided in an exhaust passageway of the internal combustion engine, and feedback control is executed to make the air/fuel ratio of the engine coincident with a theoretical air/fuel ratio. In this internal combustion engine, air/fuel ratio learning and vapor fuel concentration learning are implemented for restraining an error in the air/fuel ratio due to deteriorations of the fuel injection valve and of the air/fuel ratio sensor with a passage of time, and also fluctuations in the air/fuel ratio which occur due to the error in the air/fuel ratio with a change in the concentration of the vapor fuel. For instance, Japanese Patent Application Laid-Open No.8-177572 discloses a contrivance that an air/fuel ratio correction quantity of the feedback control when the above two learning processes are not completed, is set larger than when these learning processes are completed.
In the conventional internal combustion engine, the air/fuel ratio sensor such as an oxygen sensor is disposed normally in the exhaust passageway, and an actual air/fuel ratio is detected based on an output signal of this sensor. Then, the fuel injection quantity etc is feedback-controlled so that the air/fuel ratio of the air/fuel mixture becomes a target air/fuel ratio specially calculated. At this time, even when the air/fuel ratio becomes rich with an execution of purging the vapor fuel, the feedback control is performed so that the air-fuel mixture comes to have the target air/fuel ratio. The oxygen sensor described above, however, makes such a detection that the target air/fuel ratio (A/F) is in the vicinity of, e.g., a theoretical air/fuel ratio (A/F=14.5). In the case of the lean-burn where the air/fuel ratio is larger than this theoretical air/fuel ratio, it is impossible to highly accurately detect a change in the air/fuel ratio due to the purging. Besides, it is impossible to precisely calculate an index (e.g., a concentration of the vapor fuel) for controlling the purge quantity of the vapor fuel which has hitherto been calculated from the output of the air/fuel ratio sensor in the prior art.
The reason for this is that since the accuracy of the air/fuel ratio sensor is not high in the lean-burn state, it is difficult to control the concentration learning in accordance with the air/fuel ratio in the lean-burn state, and it is impossible to obtain the purge quantity. As a result, especially during the operation, a concentration of the purging might change due to a purge-cut and a high-load operation. Hence, if the purging is executed in a state where the purge quantity (concentration) is unobvious, the combustible air-fuel mixture in the vicinity of the spark plug becomes excessively rich during, e.g., a stratified charge combustion operation, and there might be a possibility in which an misfire occurs, and the combustion becomes unstable.